Pretreatment of aluminum surfaces

ABSTRACT

A process for preparing the surface of aluminum products to receive colored coatings comprises a substantially pollution free, non-chromate pretreatment and aluminum conversion coating process. The process provides improved coating quality, increased process stability, increased throughput and extended operating life of the processing baths by controlling the electrical resistance of the deionized water used for washing the aluminum product being processed to at least about 50 k ohms and controlling the manner of washing between each step of the process.

This application is a continuation of and claims the benefit of U.S. patent application Ser. No. 10/768,825, filed Jan. 30, 2004.

The invention is directed to a process for preparing the surface of aluminum to receive colored coatings. More particularly it is directed to a substantially pollution free, non-chromate pretreatment and aluminum conversion coating process which provides improved coating quality, increased process stability and extended operating life of the processing baths.

BACKGROUND

It is well known that it is particularly difficult to apply colored finishes to aluminum surfaces because of the oxide coating which naturally forms on the aluminum surfaces soon after it is formed. Accordingly, aluminum or aluminum alloy surfaces are routinely treated by applying an intermediate corrosion resistant conversion coating to the surface. A broad range of subsequent coatings can then be readily applied to the conversation coating to produce an acceptable, blemish free new surface. A common technique is to clean the aluminum surface and then apply a acid based hexavalent chromium composition to that clean surface. The chromium conversion coatings have been readily accepted because they are generally corrosion resistant and provide excellent retention of subsequent coatings. However, because of the extreme toxicity of chromium compounds and ecological problems resulting from waste disposal from aluminum treatment plants, the industry has replaced chromium conversion coating with chromium free systems.

A significant advance has been the use of potassium permanganate based conversion coating systems, such as developed by SanChem. Representative of such systems is the use of SanChem CC3400, a KMnO₄ based compound which is chromium, cyanide and fluoride free. A typical SanChem treatment composition comprises immersion of the aluminum product in a 10% solution of SanChem CC3400+1.7 oz activator @ 135-140° F. for 1.5-2 min to produce a gold color on the aluminum; longer time provides a thicker coating with a darker yellow color. A preliminary step is cleaning the surface prior to the permanganate dips. If an alkaline cleaner is used it is usually followed by a deoxidizer or a mild, nitric acid based solution. Generally speaking the SanChem process comprises degreasing, an alkaline wash at 160° F. (such as SanChem 500, —a mildly alkaline phosphate cleaner), deoxidizing at 125° F. using SanChem 1000 which is a nitric acid based deoxidizer free of fluorides, chromates and heavy metals, forming an oxide film by immersion in DI Water @ 210° F. and a three step sealing process (SanChem 2000+SanChem 3000+SanChem 4000).

SanChem has several patents directed to permanganate conversion coatings of which the following are representative:

U.S. Pat. No. 4,711,667 describes degreasing the aluminum surface using mineral spirits followed by an alkaline wash (NaOH, alkaline NaNO₃, HF, Na₂CO₃ or borax). This is followed by a water wash and then immersion in a bath of KMnO₄ with borax, sodium benzoate or sodium carbonate, typically for 1 minute at 155° F., followed by a water rinse. All solutions were preferably silicate free.

U.S. Pat. No. 4,988,396 is directed to aluminum alloys with 1% Cu. After the surface is degreased it is deoxidized using a 10% nitric acid solution (85° C., for 20 min.) followed by a deionized (DI) water rinse and immersion in 195°-212° F. DI water for 5 minutes to form a boehmite layer followed by the permanganate treatment.

U.S. Pat. No. 5,437,740 has several examples for anodizing aluminum and conversion coating of aluminum castings. Room temperature DI water washes after several of the treatment stages are utilized. Deoxidizing is performed using 70% HNO₃, 2.4% HF, 27.5% water.

U.S. Pat. No. 5,707,465 uses a permanganate solution containing hexavalent chromium. The aluminum material is degreased (using an organic solvent and a non-ionic detergent), rinsed with DI water, deoxidized using 10% HNO₃ at 70° F. for 1 minute and then rinsed with DI water. It was then treated with various different permanganate solutions.

U.S. Pat. No. 6,087,017 is directed to a corrosion resistant wax polyester film for aluminum. However, the pretreatment process is relevant. The aluminum panel is cleaned using a mild alkaline cleaner at 150-160° F. for 3 minute, rinsed in DI water, deoxidized in 10% HNO₃ and 3% sodium bromate at 120° F. for 5 minutes followed by a DI water rinse. This can then be followed by the application of the permanganate conversion coating at 150° F. for 1 minute.

U.S. Pat. No. 4,883,541 discusses the prior use of acidic deoxidizers in combination with HF. The claimed invention is then directed to acidic deoxidizers, particularly 10% HNO₃ with 3% sodium bromate or iodate.

U.S. Pat. No. 5,192,374 is directed to a treatment following the deoxidation step. The initial cleaning steps use an alkaline cleaner (Chemidize 740) at 71° C. for 3 min. followed by a rinse for 1 min in DI water, deoxidizing at 30° C. using 10% HNO₃₊₃% sodium bromate, rinsing with DI water for 1 min. and then immersion in boiling DI water for 5 minutes.

U.S. Pat. No. 5,417,819 is directed to compositions for desmutting aluminum surfaces comprising the use of 10-100% nitric acid and 15 gr/liter of a fluoride, the balance being water with the possible addition of H₂SO₄ and/or H₃PO₄. The preferred source of fluoride ion is ammonium bifluoride. The surface is first treated with an alkaline cleaner solution (A31K) at 140° F. It is then immediately rinsed with water, preferably DI water. The surface is then subjected to electro-brightening followed by a DI water rinse. Desmutting is then performed for 0.5-2 min @ 60-10° F. followed by a water rinse.

U.S. Pat. No. 6,123,782 is directed to a process for forming a coated aluminum product comprising cleaning the surface using a nonchromated, nonsilicated alkaline cleaner followed by a hot water rinse, deoxidizing at up to 120° F. using a nonchromated deoxidizer such as SanChem 1000 (10% nitric acid, 3% sodium bromate) and then immersion in boiling deionized water for 5-10 minutes.

Prior available processes still suffer from several recurring problems which are not addressed by a mere optimization of the process variables or use of alternative compositions. Initial operation of the processes set forth in each of the above references gives generally acceptable results. However, after a short period of continuous operation the quality of the surface produced, or subsequently applied surface, becomes unacceptable due to streaks, non-uniform appearance and subsequent fading. The treatment baths have to be dumped frequently and replaced with fresh baths in order to maintain acceptable coating properties. Prior techniques and/or solutions appear to include the existence of unrecognized and unaddressed variables which contribute to a rapid decrease in product quality. Improving the process quality is not a mere matter of optimizing the operating conditions. Applicant has discovered that there is a necessity to scrupulously process, reprocess and filter the DI water to remove interfering ions which may be in the feed and rinse water feed streams, a need to limit the time and temperature of exposure of the aluminum to DI water and the importance of subsequent use of both spray and immersion treatments. Pietschmann, J. and Jehn, Hermann (Powder Coatings, Pre-Treatment and Quality Control of Aluminum for Architectural Applications, (Galvanotechnik, D-88348 Saulgau, 91, (2000) Nr. 9)) addressed the various steps of the conversion coating process, namely degreasing, pickling/etching, acid dipping if alkaline pickling is used, conversion coating and drying. The authors states that:

-   -   “Between the individual steps, sufficient rinsing is necessary.         Rinsing is a diffusion controlled procedure. Not only the amount         of water, its purity and the temperature are decisive, but also         the duration of rinsing is important. The type of rinsing         technique, dipping or spraying, also influences the result”         However, the criticality of the washing steps, scrupulous         reprocessing of the water and the use of DI water at ambient         temperatures was not appreciated.

DETAILED DISCUSSION

A preferred process, incorporating features of the invention, comprises:

-   -   a) a cleaning step using a commercially available alkaline         composition in the manner and under conditions generally         suggested by the manufacturer,     -   b) use of 10% or greater HNO₃ as a deoxidizer with a 3% fluoride         solution (KF preferred) at room temperature, and     -   c) application of a SanChem conversion coating (SanChem 3400)         under conditions and in a manner substantially the same as         recommended by the manufacturer.

The improvement comprises the use of extensive DI water washes between each step of the process. These water washes include a combination of spray and immersion washes that appear to be critical to decontaminating the treated surface between each step to eliminate trace amounts of prior applied materials. The DI water is preferably continuously reprocessed and filtered to remove undesirable ions and maintain the resistively within defined limits. Care is also taken to not use DI water at elevated temperatures so as not to react with the surface (i.e. oxidize the surface) to form boehmite.

More specifically a process incorporating features of the applicant's invention comprises processing through a series of wash tanks as follows:

-   -   1) The aluminum part is immersed (agitation optional but         preferred) in a first water bath containing a non-silicated,         non-caustic, alkaline cleaner (such as US Specialty Color Corp         Specialty 740), under conditions recommended by the supplier,         namely 6-8 oz of the cleaner/gal at 140-165° F. (preferably 155°         F.) for 3-7 min.     -   2) The rinse is followed by a 20-30 sec atomized spray rinse         using DI water. The aluminum part is then immersed in DI water         at ambient temperature for 45-60 sec, agitation optional. The         rinse is followed by a 20-30 sec atomized spray rinse using DI         water.     -   3) The aluminum part is immersed in DI water/Nitric acid (30% by         Vol)/KFI (6-8 oz/gal) at ambient temperature for 15-60 sec. The         rinse is followed by a 20-30 sec atomized spray rinse using DI         water which drains into that dip tank.     -   4) The piece is then exposed to a 45-60 sec atomized spray rinse         using ambient DI water, a 45-60 sec immersion in ambient DI         water and a 20-30 sec atomized spray rinse using DI water.     -   5) The aluminum piece is then immersed in DI water/SanChem         CC3400 (10% by vol)+SanChem CC3400 Activator (1.7 oz/gal) at         140-165° F. (preferably 155° F.) for 2.5-4 min. Agitation         optional. The appearance of the product is light yellow.     -   6) The immersion is followed by 20-30 sec atomized spray rinse         using DI water, a 30-45 sec immersion in DI water and then a         20-30 sec atomized spray rinse using DI water.

It is preferred that all process equipment and contact surfaces (tanks, racks, baskets, hangers, etc.) are non-conductive, non-reactive material such as polypropylene, polyethylene or Teflon coated surfaces so that none of the baths or wash solutions can leach any contaminants from the processing equipment. All of the DI water feed to the system as well as the DI water spray and dip tank contents used in each wash stage may begin with city and/or well water which is filtered through a initial water purification system consisting of a) 0.5 micron particulate filter, b) a D.I mixed-bed canister and c) a 0.5 micron particulate post filtration at a rate of 10 gals/min upon demand. Once the DI water has been processed through the initial water purification system and enters the aluminum treatment system it is preferably collected and continuously filtered serially through a) a 0.5 micron particulate filter, b) a carbon filtration canister, c) a cation bed, d) two anion beds, e) a DI mixed bed canister and f) a final 0.5 micron particulate filter at a rate of from about 3 to about 10 gal/min to maintain a water quality of at least about 50 k ohms-cm. A typical DI immersion tank contains 320 gal of DI water so that the content is recycled approximately at least every 1.75 hours. Alternatively, a typical conversion coating system could treat the rinse water simply as a waste steam. Using a three stage cascading method of rinsing, the impurities from the cleanest rinse overflow to the medium purity rinse and from that into the dirtiest rinse. The dirtiest rinse is treated by adjusting the pH and ORP but is discharged to the sewer on a daily basis. In any event, the processed DI water entering each washing stage is maintained at a resistance of at least about 50 k ohms and at ambient temperature to minimize oxidation which can occur when clean aluminum surfaces are contacted with DI water at elevated temperatures.

Besides controlling the quality of the DI water used throughout the system, the chemical makeup of the water feed to the system and subjected to deionization can also have an influence on the quality of the conversion coated product. A typical analysis of the feed water entering the system is given in Tables 1 and 2. Table 3 lists the analysis of that feed water returned to the system following filtering and RO treatment. Silica, an ion known to present aluminum conversion coating processing problems, while present in the feed water, has been eliminated by the filter and deionization process, and as such the water treatment process contributes significantly to the production of quality coatings.

It has been found that operation of a process incorporating features of the invention results in significantly increased quantities of acceptable finished product which is free of streaking, spotting, peeling and discoloration. Additionally, the life of the various solution baths (the time before the quality of the treated product is unacceptable) is increased to about 480 hours of production, resulting in a significant increase in process throughput and a commensurate reduction in the down time for cleanup and bath replacement as well as a reduction in the cost for replacement bath solutions. Still further, the frequency of discharge of bath contents, which contain EPA regulated pollutants, into the surrounding environment is significantly reduced. TABLE 1 FEED WATER ANALYSIS Turbidity (Method 180.1) 0.1 NTU Turbidity after N.M filtration Conductivity (Method 151.0 MMHOS/CM Fst TDS by 143.7 Color (Method 2120C) 0.0 Color after N.M Acidification pH(Method 150.1) 7.7 Tannins N.D. (Concentrations reported as mg/L (PPM) unless otherwise reported)

CATIONS (Method 200.7) ANIONS (Method 300.0) As As As As Element CaCo3 Ion CaCo3 Calcium (Ca) 9 22.5 Chloride (CI 7.9 11.1 Magnesium (Mg) 4.4 18.1 Nitrate/Nitrite 1.2 4.3 Sodium (Na) 13.9 30.3 Sulfate (SO4) 4 4.2 Potassium(K) 3.7 0.1 Bi-carbonate 55.3 45.3 Barium (Ba) 0.3944 Fluoride (F) 0.2 0.50 Iron (Fe) ND Silica (SiO₂) 53.1 Strontium (Sr) 0.1 0.1 Manganese (Mn) ND Copper (Cu) 0.116 Zinc (Zn) 0.11 Mg/L GPG Cations 75.7 4.42 Anions 69.7 4.08 Hardness (CaCO3) 40 2.4 Additional N.D. μg/L Aluminum by ICP

TABLE 2 FEED WATER ANALYSIS Inorganics Analyte Method Result Units PQL Dilution DLR Antimony (Sb) EPA200.8 ND μg/L 2 1 2 Arsenic (As) EPA200.8 2.0 μg/L 2 1 2 Barium (Ba) EPA200.8 34 μg/L 5 1 5 Beryllium (Be) EPA200.8 ND μg/L 1 1 1 Cadmium (Cd) EPA200.8 ND μg/L 1 1 1 Chromium - EPA200.8 1.0 μg/L 1 1 1 Total (Cr) Cobalt (Co) EPA200.8 ND μg/L 50 1 50 Copper (Cu) EPA200.8 ND μg/L 5 1 5 Lead (Pb) EPA200.8 ND μg/L 5 1 5 Mercury (Hg) EPA200.8 ND μg/L 10 1 10 Nickel (Ni) EPA200.8 ND μg/L 10 1 10 Nitrate (NO3) EPA200.8 4.0 mg/L 1 1 1 Selenium (Se)— EPA200.8 ND μg/L 2 1 2 (Total) Silver (Ag) EPA200.8 ND μg/L 10 1 10 Thallium (Tl) EPA200.8 ND μg/L 1 1 1 Vanadium (V) EPA200.8 ND μg/L 10 1 10 Zinc (Zn) EPA200.8 68 μg/L 50 1 50 μg/L = micrograms/liter (ppb) PQL: Practical Quantitation Limi DLR: Detection Limit for Reporting : PQL × Dilution ND: None Detected at DLR

TABLE 3 POST RO TREATMENT Ppm Ion Ppm CaCO₃ Cations Aluminum 0.21 1.18 Barium BDL NA Cadmium BDL NA Calcium 0.19 0.5 Chromium(+3) BDL NA Copper BDL NA Iron BDL NA Lead BDL NA Magnesium 0.05 0.21 Manganese 0.08 0.15 Nickel BDL NA Potassium 0.48 0.61 Sodium 0.16 0.35 Zinc BDL NA TOTAL 2.96 CATIONS Strong Anions Chloride 02.1 0.30 Chromium(+6) BDL NA Fluoride BDL NA Hydroxide BDL NA Nitrate 11.2 9.07 Phosphate BDL NA Sulfate BDL NA SUB. TOTAL 9.37 Weak Anions Bicarbonate BDL NA Carbonate BDL NA Cyanide BDL NA SUB-TOTAL 0.00 TOTAL 9.37 ANIONS

pH 4.21 Units (by meter) Silica BDL mg/L SiO₂ TOC BDL mg/L COD NA mg/L Carbon NA Ml/gram Isotherm Conductivity 54.0 μmhos/cm Color clear

Process Information: System Flowrate (gpm) 6 Operating Temp. (° F.) 120 Hours Operated/Day 16 Days Operated/Week 5 Cleaner/System Type aqueous Water Source DI Water Quality 2 MΩ-cm (Required water quality - 50 Kohms-cm)

Besides providing improved quality conversion coated aluminum products, the process incorporating features of the invention has also been found to generate less waste products, and the waste products generated are easier to handle.

Typical waste streams generated by the conversion coating process and DI recycling procedure are:

-   -   1) depleted coating/processing tank contents,     -   2) processing tank sludge (solids or slurry deposited in the         bottom of processing tanks),     -   3) materials filtered from the DI water, and     -   4) discharge generated by reactivating the deionizing filters.

By employing the DI reprocessing described herein, it has been discovered that

-   -   1) The processing tank contents have an extended processing         life,     -   2) The amount of sludge generated is decreased, and can be         readily and safely collected, dried and disposed of as a safe         solid waste, and     -   3) Ionic materials and organic materials discharged from the         filtration system can be processed in a safe manner and the         particulate filters can be dried and discarded as safe solid         waste.

For comparison purposes, set forth below are four examples of procedures evaluated for applying a non-chromate conversion coating. These examples, while not using applicant's washing and DI water cleaning and recycling procedure, were less effective experimental attempts to obtain a suitable end product. As a result they all suffer from operating deficiencies including varying amounts of streaking, spotting, peeling and discoloration to the finished (treated) product and limitations on bath operating life. Typical prior baths and treatment procedures was found to be capable of processing only about 60,000 ft² of acceptable product before the treatment baths had to be discarded and new solutions prepared.

EXAMPLE 1

A four (4) stage system for cleaning and conversation coating aluminum alloy was utilized comprising:

Stage 1—Sanchem 560 Mild acid cleaner @ 150° F. for 10 min

Stage 2—Hot D.I. Rinse @ 140° F. for 30-60 sec.

Stage 3—Sanchem CC3400 conversion coating @ 160° F. for 3 min

Stage 4—Hot D.I. Rinse @ 140° F. for 30-60 sec.

All process equipment, (hooks, baskets to tanks) was stainless steel. Appearance of the processed aluminum parts was poor, coverage was splotchy and streaky, film build was irregular and was found to rub off. The process was not adequate to remove oils and dirt on the surfaces.

EXAMPLE 2

An alkaline wash was added to the front end of the process to better remove oils and dirt. The system includes six (6) process steps.

Stage 1—Sanchem 8104 silicated alkaline cleaner @ 120-130° F. for 10 min.

Stage 2—D.I. Rinse @ 120-130° F. for 45-60 sec.

Stage 3—Sanchem 560 Mild acid cleaner @ 150° F. for 1-2 min.

Stage 4—Hot D.I. Rinse @ 140° F. for 45-60 sec.

Stage 5—Sanchem CC3400 conversion coating @ 160° F. for 3.0-3.5 min.

Stage 6—Hot D.I. Rinse @ 140° F. for 45-60 sec.

The processed aluminum alloy parts were slightly improved in appearance for a short period of time (about 3 hours), but the life of bath solutions was not adequate (about 5 hours) due to poor product surface quality. Coating voids occurred at hang points on work pieces; streakiness and rub off still existed.

EXAMPLE 3

The make-up of stage 3 solution was changed in an attempt to improve cleaning.

Stage 1—Sanchem 8104 silicated alkaline cleaner @ 120-130 F for 10 min.

Stage 2—D.I. Rinse @ 120-130° F. for 45-60 sec.

Stage 3—Sanchem 6500 acid cleaner @ 150° F. for 1-2 min Stage 4—Hot D.I. Rinse @ 140° F. for 45-60 sec.

Stage 5—Sanchem CC3400 conversion coating @ 160° F. for 3.0-3.5 min Stage 6—Hot D.I. Rinse @ 140° F. for 45-60 sec.

The Sanchem 6500 solution contained approximately 3% nitric acid in addition to CC6500 surfactant. This was an improvement over the process of Example 2. However the appearance of product was still unacceptable. The stainless hangers and baskets were reacting at a different rate than the aluminum alloys thereby causing voids at contact points. Oil was still not adequately removed.

EXAMPLE 4

The silicated alkaline cleaner (stage 1) was changed to a non-silicated alkaline cleaner. All baskets and racks were changed from stainless steel to aluminum. The steps were as follows:

Stage 1—Specialty 740 non-silicated, non-caustic alkaline cleaner @ 150° F. for 3-7 min.

Stage 2—D.I. Rinse @ ambient for 45-60 sec.

Stage 3—De-Ox 30% Nitric Acid w/Potassium Fluoride @ ambient for 15-60 sec.

Stage-4—Spray rinse w/D.I. @ ambient for 45-60 sec.

Stage 5—D.I. Rinse @ ambient for 45-60 sec.

Stage 6—Sanchem CC3400 conversion coating @ 160° F. for 2.5-3.0 min.

Stage 7—D.I. Rinse @ ambient for 45-60 sec.

These changes resulted in a significant improvement in the quality of the surface of conversion coated products over the methods, of Example 1-3. The products were consistently finishing without the splotchy, streaky appearance and the coating was no longer thickening excessively, thereby eliminating flaking and rub off of the surface coating. Switching to aluminum hangers and baskets also eliminated witness marks. However, the bath life of the chemical solutions was not acceptable. After about 72 hours (or processing about 76,000 ft² of products) streaking re-appeared and filtration cost increased significantly. The system baths had to be changed ever 120 hours (or after processing only 127,000 ft² of aluminum parts) as product quality was no longer acceptable.

The filtration system was then modified to increase its ability to handle the ionic loading, pump sizes were increased to improve flow through the recycle system, the efficiency and filtering capacity of the filters were increased, inert liquid contacting surfaces (tanks, hangers, baskets, etc.) were installed and instrumentation was installed to monitor and control the process, it was discovered that the overall quality of the coated products was significantly increased, by these changes, the amount of treatment materials required was reduced and the cost of operating the conversion process was reduced. By improving the filtration as set forth herein, monitoring the flow rate and maintaining resistance of the wash solution at equal to or greater than about 50 K-ohms, it was discovered that the bath life could be increased to 256 hours, allowing processing of about 270,352 ft² of aluminum before product quality started to degrade. The wash solutions could still be used for an additional period, although with decreasing product quality, and had to be discarded and the treating system baths cleaned out at around 480 hours of processing time. Also, wash water flow rates were increased to 10 gpm without degrading the quality of the end product. It was also discovered, because the water processing reduced the corrosive nature of the wash solution, that CPVC plumbing materials were adequate for the purpose of transporting process water through the filtration system. However, PVDF or polypropylene can be used.

De-ionized water is naturally corrosive. While heating the rinses was expected to improve the process, because an elevated temperature would allow the parts to dry more rapidly, it was found that components retained water in cupped areas, and the remaining water would remove the finish within a couple of minutes. Accordingly, it was also unexpectedly discovered that the elimination of heated rinses significantly improved the product produced. This improvement was not offset by a slight increase in drying times which were required in the end stages of the process.

It is evident from the foregoing that there are many additional embodiments of the present invention which, while not expressly described herein, are within the scope of this invention and may suggest themselves to one of ordinary skill in the art. For example, the invention may be further improved by adding an additional chemical bath including a sealing rinse to add further corrosion resistance and improve paint and powder coat adhesion or apply, inline, a functional coating to be used in place of paint or powder coating. Further, it is not intended that processing of aluminum incorporating features of the invention be limited to the chemical compounds specifically identified by trade names. Alternatives or competitive compositions may be used. However, critical to the quality of the end product is the washing steps taken, the extensive cleaning and filtering of the DI wash solutions, and maintaining the electrical resistance of the wash solutions.

It is therefore intended that the invention be limited solely by the appended claims. 

1. An improved process for the conversion coating of aluminum products comprising the steps of: a) immersing an aluminum product in a solution for removal of dirt and oil, followed by b) immersion in an oxidizing solution, and then followed by c) immersion in a solution of a permanganate based conversion composition, the improvement comprising, following each of step a), b) and c) above, subjecting the aluminum product, upon removal from immersion in said solution, to at least one spray wash with deionized water and at least one immersion in deionized water, the deionized water being processed or continuously reprocessed through a series of filters and ion beds to maintain an electrical resistance of at least about 50 k ohms, said deionized water maintained at a temperature suitable to not form boehmite.
 2. The process of claim 1 wherein the aluminum product, after each of steps a), b) and c) is exposed to i. a first spray wash with deionized water, ii. followed by immersion in deionized water, followed iii. by a second spray wash with deionized water.
 3. The process of claim 1 wherein the deionized water is maintained at an electrical resistance of at least about 50 k ohms by passing it through at least a 0.5 micron filter, a carbon filter, a cation bed, an anion bed and a DI mixed bed.
 4. The process of claim 1 wherein the deionized water is processed serial through a 0.5μ filter, a carbon filter, a cation bed, two anion beds, a DI mixed bed and a 0.5μ filter.
 5. The process of claim 1, wherein at least about 270,000 ft² of exposed surface of aluminum parts is processed before streaking, spotting, peeling or discoloration of the surface appears.
 6. The process of claim 1, wherein aluminum parts can be processed for at least 480 hours before unacceptable streaking, spotting, peeling or discoloration of the surface appears.
 7. An improved method of applying a conversion coating to aluminum parts, said aluminum parts being exposed to treatment baths for a cleaning the parts, b) oxidizing the exposed surface of the parts and c) conversion coating the parts, wherein the parts are spray washed and immersed in deionized water between each treatment bath exposure, the deionized water provided at a temperature sufficiently low so as to not form boehmite and having an electrical resistance equal to or greater than about 50 k ohms.
 8. The improved method of claim 7 wherein the deionized water is maintained at a resistance of at least about 50 k ohms by continuously recirculating the water through at least a 0.5μ filter, a carbon filter, a cation bed, an anion bed and a mixed bed at a flow rate from about 3 gal/min to about 10 gal/min.
 9. The improved process of claim 1 further including a deoxidizing step between step a) and step b) using a solution of nitric acid and a fluoride compound. 